www.farfarfar.com has some good (though not error-free) information regarding line4. It also has a reasonably good program you can practice against, and most importantly, a number of links to other sites related to line4.
(hide) If you only look at some of the discussion boards on a regular basis you can add them to your favourite boards list by going to the Board page and clicking "Add to my favourite boards". (pauloaguia) (show all tips)